1. Field of the Invention
The present invention relates to a display apparatus, and more particularly to an electroluminescent (hereinafter referred to as EL) display apparatus with a matrix display panel including EL elements.
2. Description of the Related Art
An EL display apparatus with organic EL elements, that is, display elements which emit light spontaneously and which are arranged in a matrix pattern, is known conventionally. A passive matrix type EL display apparatus is available as such an EL display apparatus. In this type of EL display apparatus, parallel cathode lines serve as common lines, while parallel anode lines which are perpendicular to the cathode lines and which are made of ITO (indium tin oxide) serve as data lines. An organic EL layer is arranged between the set of the cathode lines and the set of the anode lines. A positive voltage is applied to the data lines in each of cathode selection periods, thereby driving organic EL elements located at the intersections of the common lines and the data lines. The display apparatus displays an image which corresponds to the voltage applied to the data lines. In the case of the passive matrix type EL display apparatus which displays an image by driving such organic EL elements, the larger the number of common lines and/or the number of data lines, the shorter the selection period (duty H) per pixel. The period of time over which the organic EL layer keeps emitting light even after the application of a voltage between the set of the cathode lines and the set of the anode lines is short. In consideration of this, according to the conventional passive matrix type EL display apparatus, the instantaneous luminance of the organic EL layer of each pixel during the selection periods is intensified so that the organic EL layer apparently emits light over 1 frame period. The organic EL layer can emit light at a high instantaneous luminance by applying a high voltage to the organic EL layer. In this case, however, the organic EL layer can easily deteriorate.
In the passive matrix type EL display apparatus, the larger the number of common lines and data lines, the more possibility of the occurrence of crosstalk. This makes it difficult to enable the passive matrix type EL display apparatus to display a highly precise image.
Proposed as a display apparatus free from the above-described problems is an active matrix type display apparatus which includes, as shown in FIG. 22, pairs of thin film transistors which confer a voltage storing capability on the pixels. Each of the pairs of thin film transistors consists of a selection transistor T1 and a drive transistor T2. The selection transistor T1 is connected to a data line DL for supplying a data signal and a gate line GL for supplying a gate signal. The gate electrode of the drive transistor T2 is connected to the selection transistor T1. The source of the drive transistor T2 is connected to a constant voltage line VL. In this display apparatus, as shown in FIG. 23, the thin film transistors T1 and T2 are formed in a pixel area on a glass substrate 101, and the gates of the thin film transistors are covered with a gate insulation film 102. In an area adjacent to the thin film transistors T1 and T2, a transparent anode electrode 103 is provided on the gate insulation film 102. The transparent anode electrode 103 is connected to the drain of the drive transistor T2. A passivation film 104 covers the thin film transistors T1 and T2. A contact hole extending up to the transparent anode electrode 103 is formed in that part of the passivation film 104 which is located on the transparent anode electrode 103. An organic EL layer 106, which absorbs the energy generated due to the recombination of electrons and holes when a current flows, is deposited in the contact hole 105 extending up to the transparent anode electrode 103. A cathode electrode 107, which reflects visible light and which extends over a plurality of pixels, is laminated on the passivation film 104 and the organic EL layer 106. In this EL display apparatus, the efficiency of the injection of carriers into the organic EL layer 106 depends on the ionization potential of the anode electrode 103 and the electron affinity (the work function) of the cathode electrode 107. In order to improve the light emitting efficiency of the organic EL layer 106 which depends on the carrier injection efficiency, the cathode electrode 107 is formed using a material whose work function is low. Since the cathode electrode 107 is normally formed of a metal such as magnesium whose work function is low, the cathode electrode 107 reflects light having a wavelength in a range of wavelength of light which the organic EL layer 106 emits. Due to this, in the above EL display apparatus, the light emitted by the organic EL layer 106 travels through the anode electrode 103 and the substrate 101. The organic EL layer 106 is arranged so as not to overlap the thin film transistors T1 and T2. The purpose of thus arranging the organic EL layer 106 is to prevent the light emitted by the organic EL layer 106 from entering the thin film transistors T1 and T2. If the emitted light entered the thin film transistors T1 and T2, unnecessary photoelectromotive force would be generated in the channel regions of the thin film transistors T1 and T2, which entails the possibility of the thin film transistors T1 and T2 malfunctioning.
In the active matrix type EL display apparatus described above, the light emitting area of each pixel in which a part of the organic EL layer 106 is located is limited to an area in which the thin film transistors T1 and T2 are not located, and therefore the ratio of the light emitting area to the pixel area is small. If the light emitting area is enlarged and if a voltage applied to the organic EL layer 106 is intensified to attain the desired luminance, the organic EL layer 106 will be considerably deteriorated. The cathode electrode 107 is made of a metal, while the organic EL layer 106 is made of an organic material. Hence, it is difficult to join the cathode electrode 107 and the organic EL layer 106 together in a preferred condition. As time passes, a gap can easily occur between the cathode electrode 107 and the organic EL layer 106, which entails the possibility that the organic EL layer 106 may become emit no light. The organic EL layer can emit light at the same luminance as that of an inorganic EL layer even when the organic EL layer is formed as thin as 40 nm to 250 nm. The thicker the organic EL layer 106, the higher an effective voltage/current for causing the organic EL layer to emit light at the desired luminance. This limits the range of value at which the thickness of the organic EL layer can be set Meanwhile, the thickness of the passivation film 104, which covers the thin film transistors T1 and T2, is set at such a value as to prevent the occurrence of a parasitic capacitance in the thin film transistors T1 and T2. Owing to a difference in thickness between the passivation film 104 and the organic EL layer 106, a step is present on the upper surfaces of the passivation film 104 and organic EL layer 106. There is the possibility that the cathode electrode 107 may break at that step. If the cathode electrode 107 breaks, the display apparatus cannot perform a display operation.